A radar device is known as a device that measures the position of an object existing at a distant point.
A radar device emits a wave, such as an electromagnetic wave or an acoustic wave, into space, and receives a wave which is reflected by an object which is an observation object and returns thereto, and analyzes the wave to measure the distance and the angle from the radar device to the object.
A weather radar device that sets a fine aerosol floating in atmospheric air as an observation object, and that measures the speed (wind speed) at which the aerosol is moving from the amount of phase rotation of a wave which is reflected by the aerosol is known among radar devices.
Further, because a laser radar device that uses light as an electromagnetic wave especially, among weather radar devices, has a very small divergence of the beam emitted thereby and can observe an object with a high angular resolution, the laser radar device is used as a wind direction and wind speed radar device (refer to nonpatent reference 1).
A conventional laser radar device emits laser light into atmospheric air, and, after that, receives laser light which receives a Doppler frequency shift according to the movement speed of an aerosol in the atmospheric air, and performs heterodyne detection on the laser light and local light, thereby detecting a Doppler signal corresponding to the wind speed.
In general, the laser light reflected from the aerosol in the atmospheric air at each altitude is split into segments by time (laser light split by time is referred to as a “range bin”), and a coherent integration at very short intervals is performed within each range bin, as shown in FIG. 15.
After that, after a Fourier transform is performed within the range bin, N incoherent integrations are performed on a pulse, as shown in FIG. 16, thereby improving the signal to noise ratio (referred to as the SNR from here on).
It is generally known that when the N incoherent integrations are performed, the SNR is improved by a factor of root N (for example, refer to patent references 1 and 2).
As a result, a power envelope as shown in FIG. 17 can be acquired.
However, it is known that in such a measurement of a wind speed as above, the coherency of the Doppler signal is weak. More specifically, it is known that the coherence time of the Doppler signal is short.
For example, it is known that when the received light of the radar device is scattered light reflected from an aerosol in the atmospheric air, the coherence time of the Doppler signal is of the order of microseconds (μs).
Therefore, a conventional laser radar device provides an improvement in the SNR by performing N incoherent integrations on a pulse, as mentioned above.
For example, a method of deriving an optimal number of integrations in order to make it possible to carry out a high-accuracy wind measurement is disclosed in patent references 1 and 2.
When the density of the aerosol is high as a result of this integration processing, as shown in FIG. 17, a high-accuracy wind measurement can be carried out.